1. Field of the Invention
The present invention relates to wireless systems such as a wireless broadband (WiBro) system used for communication with rapidly changing channel states. More particularly, the present invention relates to a cross layer optimization method for controlling a transmission bit rate of a video coder/decoder (codec) in video data transmission/reception in a wireless broadband (WiBro) system.
2. Description of the Related Art
A typical WiBro system provides a high-speed Internet service, which enables a user to access the Internet and to access various information and contents with a high transmission rate of about 1 Mbps grade through a mobile terminal anytime and anywhere, regardless of whether or not the user is stationary or in motion. Such a WiBro system is a recently developed service, which guarantees a seamless wireless Internet service within a cell radius of 1 Km using a 2.3GHz frequency bandwidth, especially when in motion at a speed of at least 60 Km/H. Such functionality enables a user to use wireless Internet at a relatively inexpensive cost. More recently, a WiBro system has been developed to a level capable of supporting mobility of speeds up to 70 Km/H and a transmission rate of up to 1 Mbps per subscriber. Moreover, if such a WiBro system were to be commercialized, it is expected that the system could support mobility of 100 Km/H and a transmission rate of up to 3 Mbps. Accordingly, when using the Internet via such a WiBro system, a user can enjoy movies whether walking or standing on the street, or traveling in a moving vehicle, and can even play online games and surf the web in a manner similar to access the Internet via a wired indoor network. The WiBro system, which is a one-step forward technology from a wireless local loop (WLL) technology of 2.3 GHz bandwidth, includes a 4th generation telecommunication field, and at the same time, has a wider field than that of international mobile telecommunication (IMT) -2000 (that is, 3rd generation telecommunications). Therefore, a WiBro system is often referred to as a 3.5th generation telecommunication technology.
A WiBro system typically includes characteristics of a conventional mobile terminal, and yet at the same time, includes characteristics of a broadband convergence network (BcN) in which communications, broadcast, and Internet are merged. Therefore, in the system, a variety of services such as online/offline, wired/wireless, and communications/broadcast, etc. are combined with each other.
As described above, mobile Internet using systems such as a WiBro system, is a technology capable of wirelessly receiving high-speed data even while in motion. Also, the mobile Internet basically provides various quality of service (QoS) levels. In other words, according to services provided by an upper layer of a mobile terminal, several QoS parameters (such as transmission bandwidths) change.
During operation of video transmission applications via a WiBro network, such as video telephony, personal broadcast etc., a WiBro system controls a modulation and coding selection (MCS) level and terminal power according to WiBro radio channel characteristic parameters, such as a carrier to interference and noise ratio (CINR), transmission power (Tx Power), etc., and raises the efficiency of the entire data throughput by an algorithm of a scheduler based on the above control.
However, the characteristics of a WiBro system using a wireless network may result in rapidly changing radio channel characteristics, traffic congestion, and a large fluctuation of the number of users within a cell, etc., thereby continuously changing an available bandwidth for transmission/reception. In other words, the WiBro system, differs from a wired network in that physical media characteristics, such as a data transfer rate, can suddenly change according to the characteristics and conditions of wireless media. Moreover, it is very difficult to predict a change in the characteristics, making the available bandwidth at a given moment difficult to predict. Such a change does not cause serious problems in the use of simple Internet search services, but may cause remarkable service degradation in terms of delay and jitter in the use of a multimedia service, such as video.
In addition, in the case where a service provider is required to provide a bandwidth guarantee, rapidly changing radio channel characteristics can cause serious service degradation in terms of QoS. Therefore, as user requirements for multimedia grow with the development of networks, the QoS guarantee has been recognized as an essential consideration in the development of communication systems.
When trying to transmit video data to the other party, a conventional WiBro terminal uses a constant bit rate (CBR) by fixing an output bit rate of a video codec. Accordingly, video data transmitted with the CBR cannot adapt itself to the sender/receiver side radio channel characteristics and WiBro network radio channel characteristic changes due to available bandwidth variations. As a result, when the radio channel characteristics of the sender side transmitting the video data deteriorate, or when the receiver side suffers from poor radio channel characteristics (even though the sender side has good radio channel characteristics), the received images may have serious degradation to image quality, or can be interrupted or even stopped. Moreover, delay and jitter characteristic degradations will become apparent, resulting in user dissatisfaction with the network.